175? data new;
176? merge class1 class2;
177? by age;
178? run;

ERROR: BY variables are not properly sorted on data set WORK.CLASS1.
Name=Mary Sex=F Age=15 Height=66.5 Weight=112 FIRST.Age=1 LAST.Age=0 _ERROR_=1
_N_=9
NOTE: The SAS System stopped processing this step because of errors.
NOTE: There were 10 observations read from the data set WORK.CLASS1.
NOTE: There were 1 observations read from the data set WORK.CLASS2.
WARNING: The data set WORK.NEW may be incomplete.  When this step was stopped
         there were 8 observations and 5 variables.
WARNING: Data set WORK.NEW was not replaced because this step was stopped.
NOTE: DATA statement used (Total process time):
      real time           7.31 seconds
      cpu time            0.01 seconds